In a wind turbine it is essential to control the pitch angle of the blades. A conventional wind turbine has a rotor, comprising a number of blades, usually three blades, and a spinner. In some turbine models, the so-called pitch regulated turbines, the pitch angle of these blades can be adjusted mechanically and dynamically by a pitch mechanism depending on the actual wind conditions. In operation, the control system of the wind turbine will adjust or control the pitch angle of the blades to ensure optimum performance of the wind turbine. This is to keep the rotational speed of the rotor within operating limits and to minimise wear of the wind turbine. Under certain circumstances, such as high or low wind speeds, the wind turbine can be taken out of service either by completely stopping the rotor or at least ensure a slow revolution speed of the rotor. This can be done by adjusting/controlling the pitch angle of the blade so that the angle of attack of the blades is diverted from the wind.
The known pitch-regulated wind turbines have a control system, which controls the pitch of the blades. This system bases control of the pitch angle on the measured wind speed by the turbine wind sensors. An anemometer placed on the nacelle, behind the rotor is an example of a frequent method used for measuring the wind speed. This position of the wind sensors is, however, far from ideal, as the wind vane will measure the wind after it has passed the rotor, when the wind turbine is in operation. The measurements are therefore heavily influenced by the turbulence generated by the rotor as well as by other aerodynamic effects caused by the nacelle. In addition, buildings, trees, and neighbouring wind turbines will significantly influence the wind speed-readings. This means, that the wind vane will transfer incorrect information to the wind turbine control system. The presently used nacelle anemometers are furthermore not capable of detecting potential damaging wind phenomena such as wind shear and potentially damaging wind inclination angles. This is a disadvantage as it is desirable to have as reliable information of the wind speed, wind shear and the wind inclination angle of the rotor as possible.
Further, if the wind turbine has more than one blade, conventionally it has three blades, the blade pitch of the blades can be controlled independently of each other.
Further, it is known to use a LiDAR to establish the wind speed upwind of the wind turbine and use that information to control the wind turbine. EP 0 970 308 disclose a wind turbine with a laser anemometer system, such as a LiDAR, it is used to determine the air velocity in front of the wind turbine. In addition, it is disclosed that the determined air velocity in front of the wind turbine can be used to control the pitch of the wind turbine blades.
An example of a LiDAR controlled wind turbine is disclosed in EP 2 025 929 A2. It describes a method for controlling the pitching of the blades based on measurements from a LiDAR. The LiDAR is mounted on the wind turbine hub and configured to measure components of the wind velocity within a predetermined portion of a planar field in front of the hub.
When using a LiDAR as disclosed in the above mentioned documents the wind velocity is measured at a substantial distance in front of the wind turbine, at least 20 meters. However, the wind direction and velocity will change within this distance; consequently the components of the wind velocity measured by a LiDAR will be different than the components of the wind velocity that actually attack the blades. Consequently, the blades are not pitched optimally.
Further, if it is raining or snowing the LiDAR cannot measure the wind velocity, because the laser ray gets blocked and/or disrupted by the rain drops or snowflakes in such a way, that no reliable information can be retrieved. When no information can be retrieved the blade cannot be pitched correctly.